1. Field of the Invention
The invention relates to digital signal processing apparatus, in particular to such apparatus for recording and/or replaying digital image or video signals with a plurality of recording heads on a rotating head mechanism in slanting tracks which extend diagonally across a recording medium such as a tape.
2. Description of the Prior Art
Given the high information densities and stringent requirements under which digital video tape recording systems operate, it is not practical to design systems in which no recording and/or playback errors occur. Typical of the sorts of problems which occur are the loss of data due to a recording or playback head becoming clogged with dirt and/or recording material from the tape, inhomogeneities in the recording layer(s) on the tape and scratches in the tape. Accordingly, known digital video tape recording systems include apparatus and methods for coping with errors which occur during the recording and/or playback of video information as part of the overall operation of such systems.
As a first level of protection against such errors, error correction codes are included with the recorded video data. If the error is a minor one then it is often possible to uniquely identify the particular piece or pieces of information that are in error and what data they should be replaced with. If the error is too severe, then such error correction cannot cope and then reliance is made upon error concealment techniques to reduce the perceivable effect of the error.
In order to facilitate error concealment, it is known to sub-sample the image data into a number of different data processing and recording channels. In this way, if an error occurs in one channel, then there will be data from the remaining channels surrounding the missing data points from the defective channel. For each erroneous pixel, a replacement pixel value can be interpolated from the surrounding pixel values from the other channels within the same field or frame or alternatively from the corresponding pixel position in preceding and/or following video fields or frames. Whilst detail is still lost from the image by such errors, the overall effect of such error concealment is to make the error less immediately perceivable.
An example of digital video tape recorder apparatus employing such an approach to error handling is described in GB-A-2 140 189. In this known apparatus, with a recording head assembly having 2n heads, where n is 1, 2 or 3, a demultiplexer demultiplexes video samples of an incoming digital television signal sample-by-sample into 2n channels for supply to the 2n recording heads and a switching arrangement for switching the connections between the channels and the heads line-by-line and possibly also field-by-field or frame-by-frame of the television signal. Although GB-A-2 140 189 mentions the idea of switching the head allocation field-by-field or frame-by-frame, i.e. a temporal demultiplexing of the video signals, it does not describe a detailed implementation of this. In practice a sample-by-sample, or spatial demultiplexing of the video signals has to date been found sufficient.
GB-A-2 140 189 describes the demultiplexing of video data into four channels for supply to each of four heads A, B, C and D. A stream of video pixels for a video field is received as a stream of pixels, pixel-by-pixel from left to right within a scan line and line-by-line. The demultiplexing is applied in a cyclical manner so that successively received pixels are applied to respective ones of the heads A, B, C and D. To facilitate concealment of errors by providing that each pixel is surrounded by eight pixels not processed by the same head, switching occurs between the heads A and C and between the heads B and D on a line-by-line basis. The result of the demultiplexing operations described in GB-A-2 140 189 is illustrated in FIG. 23. It can be seen that each line of pixels contains the sequence A, B, C, D, A, B, C, D, and so on, with, however, the sequence displaced in alternate lines by two pixel positions within that line. This simple structure always ensures that a pixel is surround by pixels from the other three heads. The demultiplexing strategy described in GB-A-2 140 189 has been found to be satisfactory where digital pixel samples are recorded directly on tape.
However, in view of the high information densities involved in image data processing, particularly as image definition increases, it is desirable that some form of data compression be performed upon the image data before it is recorded. One set of techniques for achieving such data compression involves the transformation of the image data from the spatial domain into the transform domain. Once transformed into the transform domain, the redundancy within the image data can be better exploited to yield efficient compression. The data are stored or transmitted as an encoded version of the image in the transform domain.
However, it has been found that the performance of the concealment strategy of GB-A-2 140 189 can be poor in response to certain types of drop out errors, for example where there is a longitudinal scratch on the tape surface. This can mean that blocks of data on successive tracks are faulty with the result that concealment of the errors is not possible.